In order to maintain healthy growth, plants must extract a variety of elements from the soil in which they grow. These elements include nitrogen and the so-called micro-nutrients (e.g., copper, iron and zinc), but many soils are deficient in such elements or they contain them only in forms which cannot be readily taken up by plants (it is generally believed that essential elements cannot be readily taken up by plants unless they are present in dissolved form in the soil). Nitrogen is an essential element for most plants as it plays a role in the synthesis of amino acids, proteins, nucleotides, nucleic acids, chlorophyll, co-enzymes and in the overall growth and health of the plant. To counteract such deficiencies, sources of the deficient elements are commonly applied to soils in order to improve growth rates and yields obtained from crop plants. For example, nitrate and/or ammonium is often added to soil to counteract a lack of available nitrogen.
In the field of crop science, it is well known that many cultivated crops require that the soil provide relatively large amounts of nitrogen to the plant. The notable exceptions to those plants requiring nitrogen via the soil are plants from the legume family.
Specifically, leguminous plants are unique from non-leguminous plants in their ability to fix atmospheric nitrogen into ammonia. The ability to fix atmospheric nitrogen into a useable nitrogen source for the plant obviates the need for the plant to obtain nitrogen from the soil. Nitrogen fixation, however, requires a symbiotic relationship between the leguminous plant and native bacterial within the soil. One of the most extensively studied partners in this symbiotic relationship is bacteria belonging to the genus Bradyrhizobium or Rhizobium. Gresshoff, P. (1999). Identification of Plant Genes Involved in Plant-Microbe Interactions. Stacey, G. & Keen, T. (Ed.), Plant-Microbe Interactions (4th ed.) (Ch. 6). St. Paul: APS Press.
Symbiosis is generally achieved through an exchange of complex bidirectional signaling between the plant and the microbe and the microbe and the plant. Typically, plant factors, such as flavonoids and flavonoid like substances, induce colonization of the bacteria into the root nodule of the leguminous plant. (Gresshoff, 1999). Once the bacteria has colonized the root nodule, the bacteria effect morphological changes in the plant, namely root hair curling and the development of a new root organ—the nodule. (Gresshoff, 1999). The nodule permits the establishment of a new physiological environment for the nodule inducing bacteria to differentiate into a nitrogen-fixing endosymbiont, or bacteriod, for the colonized plant. (Gresshoff, 1999).
It is well known that Rhizobial motility and chemotaxis are important attributes for strain competiveness. For example, Althabegoiti, et al., 2008, FEMS Microbiol. Lett. 282: 115-123 discusses deriving a spontaneous mutant strain from USDA 110 having increased motility which enhances nodulation when compared to its wild type strain. Further. Maier, et al., 1990, Appl. Environ. Microbiol. 56 (8): 2341-2346 discusses the role of molybdenum during the biological nitrogen fixation process. Further still, Alves, et al., 2003. Plant and Soil 252: 1-9 discusses soybean inoculants used in Brazil and the importance of competiveness for effective nitrogen fixation. Finally, Bloem, J. F., et al., 2001, Bio Fertil. Soils 33: 181-189 reports the importance of competitiveness in strain selection. In the study, the researchers use genetic engineering methods to put a reporter gene (GUS) into their index strain as a way to determine the competitiveness of strains. (Bloem, et al. 2001). As the study performed (Bloem, et al. 2001) required an extensive use of chemical staining and microscopy technology, the method reported remains an impractical approach for screening large samples of microbes.
It is an object of the present invention to provide a super competitive isolate(s) of bacteria from the genus Bradyrhizobia for colonizing leguminous plants that outperforms the colonizing ability of commercially available strains, e.g., commercial strain USDA 532C. It is a further object of the present invention to provide a super competitive isolate(s) of bacteria from the genus Bradyrhizobia for colonizing leguminous plants capable of enhancing the effectiveness at promoting leguminous plant growth in comparison to commercially available strains, e.g., commercial strain USDA 532C.